Web services include application programming interfaces (API) accessed over a network by another system. Two popular implementations of web services include representational state transfer (REST) and simple object access protocol (SOAP). In both systems, servers process requests and return responses. A web resource may be referenced using a uniform resource identifier (URI) obtainable over hypertext transfer protocol (HTTP).
Web services must be upgraded when their functionality no longer meets the requirements of the user. Generally when a new version is required, the entire web service must be wholesale changed to the new version. Naturally, the web service cannot be replaced when the web service is running, resulting in system down time.